Hydraulic dampers are routinely fitted to vehicles, in particular automobile vehicles. A pressurised hydraulic damper usually comprises a piston adapted to slide inside a tube, defining two chambers one on each side of the piston and filled with a liquid. A passage through the piston provides communication between the two chambers, cooperating with a floating valve which regulates the aperture of the passage according to the direction of flow of the liquid in the passage. This flow is caused by axial movement of the piston in the tube during extension or compression of the damper. The piston is usually fixed to one end of a piston rod whose displacement within the tube is guided axially by an obturator device mounted at one end of the tube.
The obturator devices seal an end part of the damper tube and guide axial movement of the piston rod and thus of the piston inside the damper tube. An obturator device usually comprises a guide plug, at least one seal, an expansion abutment member and means for fixing all these components to the tube, the seal or seals being sandwiched between the guide plug and the expansion abutment member.
As its name indicates, the guide plug guides axial movement of the piston rod inside the damper tube. The seal provides a seal not only between the guide plug and the tube but also between the guide plug and the piston rod. As a result there is virtually no contact between the guide plug and the damper liquid, which is usually an incompressible oil. Consequently, guidance of the piston rod by the guide plug is not lubricated by the oil in the damper. Given that the guide plug and the piston rod are made of metal, friction is generated between these two components when the damper operates. This friction heats the piston rod and the guide plug, accelerating aging of the adjoining seal, and causes wear between the piston rod and the guide plug. Aging of the seal caused by heating significantly shortens the service life of the seal. Wear between the piston rod and the guide plug increases the radial clearance between these two components and thus reduces the accuracy with which axial movement of the piston in the damper tube is guided.
The radial clearance required between the piston rod and the guide plug and the absence of lubrication of the guide surfaces cause noise due to impact between the piston rod and the guide plug during operation of the damper. The loads on the damper are not strictly axial. The radial components of these loads cause mechanical impact between the piston rod and the guide plug followed by vibration of the piston rod and giving rise to a chirping noise due to interference between the piston rod and the guide plug if the amplitude of vibration of the piston rod is greater than or equal to the radial clearance between the piston and the guide plug. The noise caused in this way is unpleasant to the user.
The use of three stacked separate components (guide plug, seal(s) and expansion abutment member) means that assembly of the obturator device is relatively time-consuming, which increases the cost of the damper.